constellation: Broadcast preference changes to all content processes (#38716)

Building on the preference observer work from #38649, we now
automatically install an observer when multiprocess mode is enabled.
This observer notifies the constellation of updated preferences, which
in turn notifies each content process so the changes will be reflected
into script/layout as expected. There's a unit test that verifies this
works correctly by checking a preference-gated WebIDL property before
and after the preference is toggled.

Testing: New unit test added.
Fixes: #35966

Depends on #38649.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/shared/constellation/Cargo.toml

@@ -31,6 +31,7 @@ net_traits = { workspace = true }
 pixels = { path = "../../pixels" }
 profile_traits = { workspace = true }
 serde = { workspace = true }
+servo_config = { path = "../../config" }
 servo_url = { path = "../../url" }
 strum = { workspace = true }
 strum_macros = { workspace = true }